How to stop dusty pc innards

I currently service my Brother in laws two pc towers in the office of his workshop and the insides get very dusty and i have to use a compressor to clean them out. Is there anything on the market that would help extract to dust as it comes in , ie case fans . The office door is shut but after about 8 months they need blowing out again.
Any ideas.

In the days of main-frame computers they used to be kept in special dust-free, air conditioned rooms but nowadays pc towers are kept in busy and dusty offices. So keeping them clean is going to be difficult.